Modern society uses a variety of materials that were not in existence a generation ago. As technology progresses, the need has increased for stronger and lighter materials of construction to make products ranging in size from notebook computers to ships at sea. In the aerospace, automotive, and construction industries, engineers have been seeking ways to make their products as light as possible, while maintaining durability. Composite materials have been able to accomplish this task well. Composites, which are defined as a combination of both a filler and resin, are known for their very high strength to mass ratio. An important class of composite materials is formulated with a plastic or a resin and glass fillers. A commonly used composite material is fiberglass.
The strength and properties of composites depends upon the combination of filler and resin. Fillers, such as glass or silica, are frequently added to composites to lessen the amount of resin needed, and, as a result, reduce the cost of the composite. Fillers also change the physical properties of composites, compared to plastics or resins alone. For example, silica or glass microsphere fillers increase the compressive strength of composites, but also, sometimes decrease the tensile strength of the composite. Glass fiber fillers, on the other hand, are known to greatly increase the tensile strength of composite products and materials. Glass fiber fillers, on the other hand, are known to greatly increase the tensile strength of composite products and materials Although composite materials have been developed with remarkable strength, the most common place where the composites fail is at the interface of the filler and the resin. It is well known in the art that this failure is caused by a weak bond between the filler and the resin phase. The separation of the filler and resin has been identified by electron microscopy which provide images of the mechanical disruption at resin-filler interfaces. It has been clearly shown that structural failure of composites is related to the mechanical separation of the resin and the filler. The interface plays a very important role in the performance of composites. The interface is responsible for transferring stress from the matrix to the fibers/fillers and, therefore, high levels of matrix reinforcement are intimately related to the behavior of the interface as a stress transfer agent. Adhesion between reinforcing agents and matrix is the main interfacial property that should be maximized in order to improve stress transference. Many ways to improve adhesion in polymer composites are currently being investigated, i.e., chemical modification of surfaces such as by the use of the above-described silane coupling agent.
To increase the strength of the bond between the fillers and the resin, chemical modifications of filler surfaces have been made to change the properties of composites. The treatment of filler surfaces to change its chemical properties is called sizing. It is well known in the art that contact between resin and filler can be improved by proper sizing of the surface with chemical modifications that are compatible with the monomers and the polymerized resin system being used.
Sizing reagents are generally bifunctional molecules. One functionality of the molecule is designed to interact with the surface of the filler. In some cases the interaction is polar or ionic in nature. For example detergents to have been used to treat silica or clay fillers. The ionic end group of the detergents binds to the surface all of the clay filler. The hydrophobic tails of the detergents interact by Van der Waal's attractions with the polymer matrix of the resin in composites.
Recent in improvements in filler technologies have been made by using bifunctional sizing reagents that are capable of covalent reaction with the surface of the filler, and that also possess functional groups that can undergo covalent bond formation with the monomers used to make the polymers of a composite matrix. One important sizing reagent is aminopropyltriethoxysilane. The reagent condenses with the surface of glass fillers to form siloxane bonds with the aminopropyl group pendant to the surface. Silica surfaces that have been treated with triethoxyaminopropylsilane are compatible with resins produced by condensation, acylation, or alkylation reactions. These resins include epoxy, polyester, and polyamide resins. Another useful sizing reagent is 3-glycidoxypropyltrinmethoxysilane. The surfaces silanized with this reagent have pendant epoxide groups that can react with amines or alcohols in epoxy, polyester, and polyamide resins.
Another class of sizing reagents are bifunctional molecules that react with the silica surface and have vinyl groups that are capable of undergoing copolymerization with vinyl monomers used in radical polymerization processes. Sizing reagents can be developed that are highly selective for the particular polymer system being used. An example is U.S. Pat. No. 6,436,476 which discloses a bifunctional vinylbenzylsilane molecule that is used to modify the surface of glass fibers. The composite is prepared by ring opening metathesis polymerization (ROMP) with certain diolefin monomers.
Although much research has been done on the surface modification of fillers used in composites, there is a need to investigate new methods for increasing the bonding between the fillers and resins of composites to form composites with improved properties and adhesion.